[130.04] Large Area Mosaics of the HST Orion Treasury Program

L. E. Bergeron, S. Casertano (STScI)

The HST Orion Treasury Program (10632, P.I. M. Robberto)
used three imaging cameras aboard HST to map a large area of
the Orion Nebula Cluster. ACS was the primary camera and
imaged a contiguous area of ~535 square arcminutes in 5
bands. WFPC2 in parallel covered nearly the same area in 4
additional bands, and NICMOS was used with parallel
Field-Offset-Mechanism (FOM) dithering to maximize the area
covered (~165 square arcminutes) in two bands, although it
was not contiguous.

In order to remove the absolute HST pointing uncertainty
(typically 1-2") and mosaic the images, the 2MASS catalog
was used as an astrometric reference. Sources were
identified in the HST images and then a weighted
least-squares solution to the shifts and orientations was
determined, internally in overlapping image areas and
against the known catalog source positions simultaneously.
This resulted in excellent relative registration and
absolute World Coordinates tied directly to the 2MASS
reference. The new Multidrizzle (Koekemoer et al, 2005)
package in STSDAS was used to combine all the data into
mosaics with appropriate shifting, weighting and
CR/bad-pixel masking. The process is described here.